Method for controlling the flow rate of fuel within a gas turbine

ABSTRACT

A valve for controlling the flow of fuel within a gas turbine engine is provided. The fuel flow control valve includes a valve block disposed within a sleeve, and apparatus for displacing one of the sleeve or valve block relative to the other. The sleeve includes an inlet port and an exit port. The valve block includes an inlet gate and an exit gate. One of the sleeve or the valve block may be displaced relative to the other from a closed position where the gates close the ports and thereby prevent fluid flow through the valve via the ports, to a plurality of open positions where the gates less than completely close the ports, and thereby allow fluid flow through the valve via the ports.

METHOD FOR CONTROLLING THE FLOW RATE OF FUEL WITHIN A GAS TURBINE

The invention was made under a U.S. Government contract and theGovernment has rights herein.

This is a division of application Ser. No. 08/635,224, filed on Apr. 17,1996, (U.S. Pat. No. 5,772,182).

BACKGROUND OF THE INVENTION

1. Technical Field

This invention applies to gas turbine engine fuel controls in general,and to gas turbine engine fuel flow control valves in particular.

2. Background Information

Fuel control valves for high performance gas turbine powered aircraftmust perform with a high degree of accuracy under a wide variety ofoperating conditions. A valve that meters too little or too much fuel tothe combustor could cause a combustor to "blowout", or could hinderreignition within the combustor. To avoid such problems, valve designconsiders the difference in pressure across the fuel control valve andthe mass flow rate of fluid through the valve. These two parameters aregenerally used to define the required performance of the fuel controlvalve within the aircraft flight envelope. The difference in pressure(Δ_(p)) "across" the valve is by consensus defined to be the differencebetween the pressure of the fuel discharging from the fuel pump lesscomponent and piping head losses between the pump discharge and thecontrol valve (P_(FPD)) and the pressure of the fuel dispensed withinthe combustor(s) less component and piping head losses between thecontrol valve and the combustor (P_(FC))

The mass flow rate of the fluid passing through the valve, on the otherhand, may be determined by the equation: ##EQU1## where W_(f) representsthe mass flow rate of the fluid, K represents a conversion factorconstant, C_(d) represents a discharge coefficient for flow exiting thevalve orifice, A_(v) represents the cross-sectional area of the valveorifice, and ρ represents the density of the fluid. The dischargecoefficient (C_(d)) is a coefficient that compensates for less thanfrictionless ideal flow through an orifice and is a function of: (1) thegeometry of the orifice relative to upstream passage geometry; and (2)the Reynolds number of the fluid passing through the orifice. TheReynolds number of the fluid passing through the orifice, in turn,accounts for the velocity of the fluid within the orifice, thedimensions of the orifice, and the kinematic viscosity of the fluid. Ininstances where the ratio of pressures across the valve (P_(FPD)/P_(FC)) is no more than six (6), the discharge coefficient (C_(d)) maybe considered a constant for a particular point within the flightenvelope. This is in part due to a relatively low fluid velocity throughthe orifice. In those instances, the mass flow rate of fluid (W_(f)),and therefore the power setting of the engine, can be readily controlledby changing only the cross-sectional area of the valve orifice (A_(v)).

In instances where the ratio of pressures across existing valves exceedsix (6), however, the discharge coefficient (C_(d)) often becomesunstable due to cavitation and cannot be considered a constant for aparticular point within the flight envelope. Specifically, at pressureratios greater than six, the velocity of the fluid passing through theorifice is great enough to cause cavitation which in turn prevents aconsistent C_(d) value from being empirically determined. Control offuel flow rate through the valve in these instances must, therefore,consider at least two variables, one of which is unstable. Under thosecircumstances accurate fuel flow control through the valve is difficultat best.

To avoid having a ratio of pressures across the fuel control valve inexcess of six (6), it is known to use a hydromechanical head regulator,which is a device designed to maintain a particular Δ_(p) across a fuelcontrol valve under all conditions. Although head regulators do providethe advantage of a constant Δ_(p) across the fuel flow control valve,they also provide several distinct disadvantages. For example, headregulators used in high pressure applications tend to be of considerablesize and weight, neither of which is desirable. Head regulators also adda second layer of complexity to the fuel control system; e.g. theyrequire sensor input to operate and a flow valve to regulate a constanthead across a metering valve. The sensors and the valves within the headregulator provide additional potential failure modes which aredifficult, if not impossible, to diagnose. Head regulators also addsignificantly to the cost of most gas turbine fuel flow control systems.In short, the advantage of a constant Δ_(p) across the control valve isoffset by several distinct disadvantages.

What is needed, therefore, is a method for accurately controlling themass flow rate of fuel within a gas turbine engine that accommodateshigh pressure differences across the valve, which does not add to theweight, cost, or complexity of the fuel flow control system.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor accurately controlling the mass flow rate of fuel within a gasturbine engine.

It is another object of the present invention to provide method forcontrolling the mass flow rate of fuel within a gas turbine engine thataccommodates large differences in pressure across the valve.

It is still another object of the present invention to provide a methodfor controlling the mass flow rate of fuel within a gas turbine enginethat minimizes the ill effects of cavitation.

According to the present invention, a method for controlling the massflow rate of fuel within a gas turbine engine is provided, comprisingthe steps of:

(a) Providing a fuel flow valve having a first and a second variableorifice, an internal cavity extending between the first and secondvariable orifices, means for selectively actuating the variable orificesfrom a closed position where the orifices prevent fluid flow through thevalve, to a plurality of open positions where the orifices allow fluidflow through the valve, and means for measuring fuel flow pressurewithin the internal cavity;

(b) Measuring the fuel pressure within the internal cavity, andquantifying the measurement as a first pressure value;

(c) Measuring the fuel pressure downstream of the valve, and quantifyingthe measurement as a second pressure value;

(d) Calculating a pressure difference across the second variable orificeby comparing the first and second pressure values; and

(e) Metering the mass flow rate of fuel by selectively actuating thevalve until the pressure difference across the second variable orificeequals a first desirable predetermined value.

According to one embodiment of the present invention, the sleeve of thefuel flow control valve is disposed within a component housing. Thehousing includes a fluid inlet means and exit means aligned with theinlet port and exit port of the valve sleeve, respectively.

An advantage of the present invention is that a method for accuratelycontrolling the mass flow rate of fuel within a gas turbine engine isprovided. The present fuel flow control valve splits the pressure dropacross the entire valve into two discrete pressure drops, and therebydecreases fluid velocity passing through either orifice. Decreasing thefluid velocity through the orifices minimizes cavitation and the C_(d)instability associated therewith.

Splitting the pressure drop across the valve into two discrete dropsalso helps prevent detrimental erosion. Cavitation can cause erosion ofhardware adjacent the cavitation path and the magnitude of the erosiongenerally increases with the level of cavitation. The present inventionhelps minimize erosion by first minimizing cavitation. The presentinvention also minimizes the detrimental effects of erosion by varyingthe amount of Δ_(p) that occurs across the inlet port versus the exitport. Providing an inlet port geometry that causes a higher percentageof the Δ_(p) to occur across the inlet port than the exit port willcause most or all of any cavitation that does occur to do so within thefuel flow control valve where erosion resistant materials may beutilized.

Another advantage of the present invention is that it provides animprovement in leakage performance through the valve. Prior art fuelcontrol valves having a single port with a large pressure drop acrossthe- port are often subject significant leakage because of: (1) thesignificant pressure difference across the port driving the fluid; and(2) pressure induced mechanical distortion which provides leak paths forthe fluid. The leakage negatively affects the performance of the valveby altering the intended flow rate. The present invention, in contrast,splits the pressure drop across the entire valve into two discretezones. The smaller pressure drop across each port results in lessleakage because: (1) the pressure difference driving the fluid is less;and (2) mechanical distortion within the valve is less.

Still another advantage of the present invention is that fuel flowthrough the valve is readily controlled. The present invention providesa pair of variable orifices which are manipulated by displacing one ofthe valve block or the sleeve relative to the other. Both orifices maytherefore be controlled by a single actuation device sensed by a singleposition control device. A person of skill in the art will recognizethat it is a considerable advantage to simplify mechanical devices andminimize the number of controls required whenever possible.

These and other objects, features and advantages of the presentinvention will become apparent in light of the detailed description ofthe best mode embodiment thereof, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view showing the component sleeve, andfuel flow control valve in cross-section. The valve is shown in a closedposition.

FIG. 2 is a diagrammatic side view showing the component sleeve, andfuel flow control valve in cross-section. The valve is shown in an openposition.

FIG. 3 is a diagrammatic top view showing the component sleeve incross-section. The valve is shown in a closed position.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a fuel flow control valve 10 for a gas turbineengine (not shown) includes a valve block 12 disposed within a sleeve14, means 16 for displacing one of the sleeve 14 or valve block 12relative to the other, and means 17 for sensing the displacement of thesleeve 14 or valve block 12 relative to the other. The sleeve 14 isdisposed within a component housing 18 attached to the periphery of agas turbine engine. The sleeve 14 is cylindrically shaped and includes apair of inlet ports 20, a pair of exit ports 22, and an internal cavity24. Each inlet port 20 is diametrically opposite the other inlet port20. Each exit port 22 is diametrically opposite the other exit port 22."O"-rings 26 positioned within grooves 28 disposed within the outersurface 30 of the sleeve 14 seal between the component housing 18 andthe sleeve 14.

The valve block 12 includes a pair of inlet gates 32 and a pair of exitgates 34. The inlet and exit gates 32,34 are separated a distancesufficient to enable communication with the inlet 20 and exit 22 ports,respectively when the valve block 12 is disposed within the sleeve 14.The geometries of the ports 20,22 (see FIG. 3) and gates 32,34 arechosen to provide flow characteristics for whatever application is athand. Specifically, different geometries can provide different flow rateof changes as the valve block 12 and sleeve 14 are displaced relative toone another; e.g., a step function change, or an exponential change, ora linear change in flow rate. In the embodiments shown in FIGS. 1-3, thevalve block 12 is cylindrically shaped and includes openings 36 disposedbetween the inlet 32 and exit 34 gates. "O"-rings 38 positioned withingrooves 40 disposed within the outer surface 42 of the valve block 12seal between the sleeve 14 and the valve block 12.

The component housing 18 includes an inlet channel 44 and an exit 46channel disposed within a bore 48 for receiving the sleeve 14. Thechannels 44,46 are connected by passage means 50,52 which enable fuel toenter the inlet channel 44 and pass out of the exit channel 46. When thesleeve 14 is received within the bore 48, each channel 44,46 forms anannulus around the periphery of the sleeve 14. The aforementioned"O"-rings 26 disposed within the outer surface 30 of the sleeve 14 sealbetween the component housing 18 and the sleeve 14.

The means 16 for displacing one of the sleeve 14 or valve block 12relative to the other is shown as an electromechanical solenoid 54 typedevice. The plunger 56 of the solenoid 54 is attached to the valve block12 and can be actuated to displace the valve block 12 relative to thevalve sleeve 14. Alternatively, the solenoid 54 can be attached to thesleeve 14 for displacing the sleeve 14 relative to the valve block 12.Other linear actuators, including a hydraulic actuator coupled with ahydraulic servo valve (not shown) may be used alternatively. Lineardisplacement of one of the sleeve 14 or valve block 12 relative to theother may be described as axial displacement.

The means 17 for sensing displacement of the sleeve 14 or valve block 12relative to the other is a linear variable displacement transducer(LVDI) 19, shown diagrammatically in FIGS. 1-3. A person of skill in theart will recognize that a variety of LVDTs 19 are available for sensinglinear displacement including magnetic, optical, and electrical devices.In all cases, the output of the LVDT 19 is calibrated to indicate theposition of the valve block 12 and the sleeve 14 relative to oneanother.

In the operation of the fuel flow control valve 10, the valve 10 maystart at a closed position as is shown in FIG. 1. In the closedposition, the inlet 32 and exit 34 gates align with the inlet 20 andexit 22 ports, respectively, thereby preventing fluid flow through theports 20,22 into the cavity 24 of the sleeve 14. The "O"-rings 26disposed between the outer surface 30 of the sleeve 14 and the componenthousing 18 prevent fuel from entering the cavity 24 of the sleeve 14 orthe component housing 18 via whatever leakage path exists, if any,between the sleeve 14 and the housing 18.

In the completely open position, the inlet gates 32 do not impede theflow of fuel 57 (see FIG. 2) entering the cavity 24 of the sleeve 14 viathe inlet port 20. Likewise, the exit gates 34 do not impede the flow offuel 59 exiting the cavity 24 via the openings 36 and the exit ports 22.Less than maximum fuel flow rate may be accomplished by displacing thevalve block 12 relative to the sleeve 14 (or vice versa) such that aportion of the gates 20,22 align with the ports 32,34, thereby impedingthe passage of flow therethrough. In all cases, a reference signal valuestored in a controller (not shown) is associated with a particular valveposition and magnitude of fuel flow. The valve block 12 is displaceduntil the LVDT 19 signal compares favorably with the reference signalvalue.

Fuel entering the cavity 24 via the inlet ports 20 is motivated by adifference in pressure between the fuel exiting the fuel pump (notshown) and the fuel within the internal cavity 24 of the sleeve 14. Apressure sensor 58 in communication with the cavity 24 is used todetermine pressure within the cavity 24. Fuel exiting the cavity 24 viathe exit port 22 is motivated by a difference in pressure between thefuel within the cavity 24 and the fuel within the gas turbine enginecombustor(s) (not shown).

Depending upon the application of the fuel flow control valve 10, it maybe advantageous to establish one of the inlet 20 or exit 22 ports toperform as a throttling orifice and the other as a metering orifice. Ifthe inlet port 20 acts as a throttle and the exit port 22 as a meter,then the pressure difference between the internal cavity 24 and thecombustor(s) (not shown) may be used to calculate the fuel flow ratethrough the entire valve 10. Measuring the fuel flow rate across onlyone of the inlet 20 or exit 22 ports eliminates whatever inaccuraciesmay be associated with including the additional orifice; e.g., portcross-sectional area inaccuracies, pressure difference inaccuracies,etc.

Although this invention has been shown and described with respect to thedetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the invention. Forinstance, the actuation of one of the valve block 12 or sleeve 14relative to the other has heretofore been described as linear actuation.In an alternative embodiment, one of the valve block 12 or sleeve 14 maybe rotated relative to the other to open and closed the ports 20,22 ofthe valve 10. Rotary displacement of one of the sleeve 14 or valve block12 relative to the other may be described as radial displacement. In theabove best mode both the sleeve 14 and the valve block 12 have beendescribed as being cylindrically shaped. In alternative embodiments, oneor both of the sleeve 14 and valve block 12 may assume noncylindricalshapes.

We claim:
 1. A method for controlling the mass flow rate of fuel withina gas turbine engine, comprising the steps of:(a) providing a fuel flowvalve having:a first variable orifice; a second variable orifice; aninternal cavity extending between said first and second variableorifices; and means for selectively actuating said variable orificesfrom a closed position where said orifices prevent fluid flow throughsaid valve, to a plurality of open positions where said orifices allowfluid flow through said valve; and means for measuring fuel pressurewithin said internal cavity; (b) measuring the fuel pressure within saidinternal cavity, and quantifying said measurement as a first pressurevalue; (c) measuring the fuel pressure downstream of said valve, andquantifying said measurement as a second pressure value; (d) calculatinga pressure difference across said second variable orifice by comparingsaid first and second pressure values; and (e) metering the mass flowrate of fuel by selectively actuating said valve until said pressuredifference across said second variable orifice equals a first desirablepredetermined value.
 2. A method for controlling the mass flow rate offuel within a gas turbine engine according to claim 1, furthercomprising the steps of:(a) measuring the fuel pressure upstream of saidvalve, and quantifying said measurement as a third pressure value; (b)calculating a pressure difference across said first variable orifice bycomparing said third and first pressure values; and (c) throttling saidfuel flow by selectively actuating said first variable orifice untilsaid pressure difference across said first variable orifice equals asecond desirable predetermined value, wherein said pressure differenceacross said first variable orifice is greater than said pressuredifference across said second variable orifice.
 3. A method forcontrolling the mass flow rate of fuel within a gas turbine engineaccording to claim 1, wherein said first and second variable orificesare connected and are selectively actuated together.
 4. A method forcontrolling the mass flow rate of fuel within a gas turbine engineaccording to claim 2, further comprising the step of:(a) providingerosion resistant material within said valve to resist the formation ofcavitation sponsored erosion within said valve.
 5. A method forcontrolling the mass flow rate of liquid fuel within a gas turbineengine, comprising the steps of:(a) providing a valve which includes:asleeve, having an inlet port, an exit port, and an internal cavityconnecting said inlet and exit ports; and a valve block, having an inletgate and an exit gate; wherein said valve has an open position wherefuel enters said valve via said inlet port, passes into said internalcavity, and exits said valve via said exit port, and a closed positionwhere said gates close said ports and prevent fuel flow through saidvalve; means for measuring fuel pressure within said internal cavity;(b) measuring the fuel pressure within said internal cavity, andquantifying said measurement as a first pressure value; (c) measuringthe fuel pressure downstream of said valve, and quantifying saidmeasurement as a second pressure value; (d) calculating a pressuredifference across said exit port by comparing said first and secondpressure values; and (e) controlling the mass flow rate of fuel byselectively displacing one of said sleeve or valve block relative to theother of said sleeve or valve block, thereby opening or closing saidinlet and exit ports via said inlet and exit gates, until said pressuredifference across said exit port equals a first desirable predeterminedvalue.
 6. A method for controlling the mass flow rate of fuel within agas turbine engine according to claim 5, wherein said valve furthercomprises:means for sensing the position of said sleeve relative to saidvalve block.
 7. A method for controlling the mass flow rate of fuelwithin a gas turbine engine according to claim 5, wherein said valvefurther comprises:means for selectively actuating said inlet and exitports and said inlet and exit gates in concert.
 8. A method forcontrolling cavitation within a gas turbine engine fuel system,comprising the steps of:(a) providing a valve for controlling the flowof fuel, said valve includinga sleeve, having an inlet port and an exitport, each having a cross-sectional area; and a valve block, having aninlet gate and an exit gate; wherein said one of said sleeve or saidvalve block may be displaced relative to the other of said sleeve orsaid valve block from a closed position where said gates close saidports and thereby prevent fluid flow through said valve via said ports,to a plurality of open positions where said gates less than completelyclose said ports, and thereby allow fluid flow through said valve viasaid ports; (b) pumping fuel through said valve, wherein fuel passingthrough said valve experiences a first pressure drop across said inletport and a second pressure drop across said exit port; (c) altering saidinlet and exit ports such that said first pressure drop is greater thansaid second pressure drop; (d) providing erosion resistant materialwithin said valve to resist the formation of cavitation sponsorederosion within said valve.